Murine leukemia viruses of various host range and disease potential have been studied biochemically. Various tissue culture cell lines have been developed. Using one of these cell lines, a new group of murine leukemia virus has been identified and characterized. Several murine leukemia viruses have been molecularly cloned as infectious DNA molecules. Using these molecules, work is in progress to determine the biochemical nature of various biological functions. Studies of several groups of murine leukemia viruses and their relationship to many inbred strains of mice have contributed significantly to our understanding of many viral diseases. Recombination between sequences of replicating virus and host cell sequences in the generation of new virus is already well known. Isolation of additional groups of viruses is dependent on the use of cell culture systems in which the viruses can be replicated. Using a Mus dunni cell line, several new viruses which were previously undetected have been isolated. Biological and biochemical characterization of some of these new viruses and their role in pathogenicity may prove useful to our understanding of disease production.